In addition to the official plugins, there are many other open source and commercial plugins listed below (in alphabetical order) :

Open source

gladstone

GladSToNe is an educational tool for learning about hi-performance audio handling for real-time transmissions. The goal of the project is to use the open source model to improve the psycho acoustics, noise shaping and speed of audio transmissions over IP. Contains a G729 codec plugin, g729enc for encoding and g729dec for decoding.

gst-buzztard

Extra interfaces for audio plugins + a few plugin using them. Contains a wrapper for buzz-machine plugins. audiodelay add an echo effect to audio, the bml plugin wraps buzz-machine plugins, fluidsynth is an audio synthesizer, and simsyn is a simple monophonic audio synthesizer.